Though keep in mind that the Strategic Resource is considered unused when the previously using unit is killed/deleted. So if you have 3 Iron, Make 1 Swordsman and then have 2 Iron. Once that Swordsman is removed from your control, you will return to 3 Iron. Resources in Civ 5 are not like most games, where you gather more over time by mining. In Civ 5 when you build a Mine on a resource you immediately gain control of that particular quantity of the resource.